refactor: use visual context for flash variant styling instead of design tokens

[amanabiy]

Description

Refactors the progress bar flash variant to use visual context styling instead of component-specific CSS classes and design tokens. This addresses the cleanup suggested in this review comment.

Potential Impact:
This change may affect builders who use variant='flash' outside of a flashbar context to control styling. However, the flash variant is intended exclusively for use within flashbar components.

For builders who need custom progress bar colors:

• Use the public design tokens introduced in PR #4194
• See the linked task (D382451383) for internal usage research and impact analysis

Related links: D382451383

How has this been tested?

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.17%. Comparing base (170fdf8) to head (e10c640).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4214      +/-   ##
==========================================
- Coverage   97.17%   97.17%   -0.01%     
==========================================
  Files         882      882              
  Lines       25824    25823       -1     
  Branches     9332     9331       -1     
==========================================
- Hits        25095    25094       -1     
  Misses        723      723              
  Partials        6        6              

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[pan-kot]

Should we deprecate this variant?

[amanabiy]

No, the "flash" variant should not be deprecated. It has a functional purpose beyond styling the color like result button isn't displayed when using the flash variant.